1. Field of the Invention
The present invention relates to a spatial displacement detection device for a probe of a scanning atomic force microscope and, more particularly, to a spatial displacement detection device using a laser beam.
2. Related Background Art
As one of surface microscopes having a spatial resolution on the atomic scale, a scanning atomic force microscope (to be abbreviated as an AFM hereinafter) has been put into practical applications. The AFM utilizes an interatomic force acting on a probe and a sample surface, and forms a sample surface image with three-dimensional pattern information by detecting the force acting on the probe and a sample while two-dimensionally scanning the sample surface.
Since the AFM measures the interatomic force between the probe and sample, it can advantageously measure not only the surface of a conductive material such as a metal but also the surface of a non-conductive material including an organic material. The arrangement of a general AFM apparatus will be explained below.
A probe normally has a sharp distal end having a radius of curvature of several hundred nm, and is formed near the free end of a thin film lever with resiliency, which is called a cantilever. An actuator (e.g., a piezoelectric element) is attached to the cantilever or a sample base to allow displacements in three-dimensional directions, so that the probe and the sample base are relatively three-dimensionally movable.
When the probe is brought close to a distance of several .ANG. or less to the sample surface, an interatomic force consisting of an attractive force as a dispersion force, and a repulsive force caused by the Pauli exclusion principle effectively acts between the probe and the sample surface, and in some cases, an electrostatic force or an adsorption force via an adsorption substance acts. The cantilever is bent in proportion to a sum of these local forces. Therefore, by detecting the bend of the cantilever, information associated with a three-dimensional pattern or physical properties on the sample surface can be obtained.
Furthermore, using a material having an electric or magnetic dipole, or a mechanism for detecting an induced electromotive force for the probe, the AFM is applied to examination of the electromagnetic nature on the sample surface or inside the sample.
In the AFM, as a method of detecting displacements of the cantilever in a direction perpendicular to the sample surface (Z-direction), an optical lever method or a laser interference method using a laser beam is popularly used in view of convenience in arranging the apparatus. In these methods, a continuously emitted convergent laser beam is irradiated onto the distal end of the cantilever, and any shift of the reflection angle of the laser beam reflected by the back surface of the cantilever is detected, thereby detecting the displacement, in the Z-direction, of the probe. Based on this displacement, for example, the distribution, in the height direction, of the sample surface is obtained.
The probe is maintained at a position in the vicinity of the sample surface by being pressed by the resiliency of the cantilever or by the attractive force with the sample surface. At this time, the distance between a support portion of the cantilever and an average horizontal plane of the sample surface is maintained constant, and the oscillating displacement of the cantilever is detected as three-dimensional pattern information of the sample surface. Alternatively, an output from a photodiode is used as a feedback control signal for an actuator, which is attached to a sample or a probe and controls a displacement in the Z-direction, so as to maintain the force acting between the probe and the sample surface to be constant, i.e., to maintain the reflection angle of a laser beam reflected by the back surface of the cantilever to be constant, and the feedback control signal is detected as a three-dimensional pattern signal of the sample surface.
In the AFM with the above arrangement, by scanning the probe on an X-Y plane with respect to the sample surface, and displaying the three-dimensional pattern signal of the sample surface in correspondence with the sample surface coordinates, a three-dimensional enlarged image of the sample surface can be obtained.
In the above-mentioned AFM, when a fine displacement detection method of a cantilever using a laser beam such as the laser interference method or the optical lever method is used, the temperature of the cantilever rises due to the energy of the laser beam, and a warp or distortion is generated by thermal expansion/contraction of the temperature-raised cantilever, thus drifting the reflection angle of the reflected laser beam due to the change in temperature independently of the three-dimensional pattern on the sample surface. In particular, when one surface of the cantilever is coated with a material having a composition different from that of the cantilever, since the cantilever has an asymmetrical composition structure, the thermal expansion/contraction difference between the front and rear surfaces of the cantilever becomes conspicuous due to different thermal expansion coefficients of the materials with different compositions, thus promoting the warp or distortion of the cantilever.
Although a focal point is formed on the distal end of the cantilever by a focusing lens so as to irradiate a laser beam onto only the distal end portion of the cantilever, some light components of the laser beam are undesirably irradiated onto the probe and the sample surface due to diffusion of the laser beam caused by the shape and size of the cantilever, diffraction of the laser beam by the cantilever, or the like. For this reason, the temperatures of the probe and the sample surface rise, and the physical properties of the sample surface may change. Depending on the types of samples, a temporary or permanent change in physical properties is caused by optical pumping, and the change in physical properties may lead to a change in form of the sample. When the change in form occurs, a three-dimensional pattern image of the sample surface, which is different from that obtained before laser irradiation, is undesirably formed.
Furthermore, in atmospheric air, an adsorption force may act between the probe and the sample surface via an adsorbent substance such as water molecules present on the sample and probe surfaces, and may account for principal components of the attractive force acting between the probe and the sample surface. However, when the temperature of the sample or probe rises upon irradiation of the laser beam, adsorbed molecules dissociate, and the adsorption force decreases. The measurement using the AFM is controlled to detect the force acting on the probe as three-dimensional pattern information on the sample surface or to maintain the force acting on the probe to be constant. For this reason, when the adsorption force between the probe and sample changes, the force acting on the probe changes as time elapses, and the resolution undesirably changes.
In addition, in a gas including atmospheric air, heat dissipation is attained by transmission of energy to gas molecules by collision of the gas molecules, in addition to a heat dissipation process due to heat conduction or heat radiation of the cantilever. However, when the measurement using the AFM is performed in a vacuum, since the density of gas molecules is very small, the heat dissipation performance of the cantilever is lowered, resulting in temperature rise. Therefore, the above-mentioned warp or distortion of the cantilever due to its thermal expansion/contraction is generated.